A
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Separation and Purification Technology 58 (2007) 179–191
Review of titania nanotubes synthesized via the hydrothermal treatment:
Fabrication, modification, and application
Hsin-Hung Ou, Shang-Lien Lo
∗
Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering,
National Taiwan University, 71 Chou-Shan Road, Taipei 106, Taiwan, ROC
Abstract
In spite of the controversy about the chemical structure and formation mechanism of titania nanotubes (TNTs), they are still gaining prominence
owing to their unique features including large specific surface area, photocatalytic potential, and ion-exchangeable ability. In view of this, a
comprehensive list of literatures on characterizations, formation mechanism, and applications of TNTs was compiled and reviewed. From a
literature survey, it is apparent that the dependence of TNT attributes on the synthesis conditions and on the post-treatments significantly dominates
the feasibility of applications. So far, studies intended for rapid formation kinetics and for modifications of TNTs are not exhaustive. That may be
the promising aspects in the following developments of TNTs.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Titania nanotubes; Hydrothermal treatment; TiO
2
1. Introduction
Nanosize materials with peculiar properties are not expected
in bulk phase and have already led to a breakthrough in var-
ious fields of science and technology. Over the past decades,
nanosize materials derived from TiO
2
have extensively been
investigated for vast applications, including solar cells/batteries,
electroluminescent hybrid devices, and photocatalysis, owing
to their peculiar chemical and physical behaviors. Moreover,
the discovery of carbon nanotubes intrigued the intensive
researches of one-dimensionalnanostructures, such as nanotube,

nanorod, nanowire, and nanobelts. TiO
2
-based nanotubes, there-
fore, attracted extensive and engrossing interest, despite the
crystalline structure still being controversial. TiO
2
-based nan-
otubes with high specific surface area, ion-changeable ability,
and photocatalytic ability have been considered for exten-
sive applications. Currently developed methods of fabricating
TiO
2
-based nanotubes comprise the assisted–template method
[1–3], the sol–gel process [4], electrochemical anodic oxida-
tion [5–10], and hydrothermal treatment [11–23]. The scenario
of fabrication approaches for TiO
2
-based nanotubes is demon-
strated in Fig. 1.TiO
2
-based nanotubes were first reported
by Hoyer [1] via the template–assisted method. Thereafter,
∗
Corresponding author. Tel.: +886 2 23625373; fax: +886 2 23928830.
E-mail address: (S L. Lo).
electrochemical anodic oxidation and hydrothermal treatment
succeeded in fabricating TNTs. Each fabrication method can
have unique advantages and functional features and compar-
isons among these three approaches have been compiled in
Table 1. Regarding the template–assisted method, anodic alu-

minum oxide (AAO) nanoporous membrane, which consists of
an array of parallel straight nanopores with uniform diameter
and length, is usually used as template. The scale of TNTs can
be moderately controlled by applied templates. However, the
template–assisted method often encounters difficulties of pre-
fabrication and post-removal of the templates and usually results
in impurities. Concerning electrochemical anodic oxidation, the
self-assembled TiO
2
nanotubes (␲-TiO
2
) with highly ordered
arrays was discovered by Grimes’ group [6], and the method
is based on the anodization of Ti foil to obtain nanoporous
titanium oxide film [5]. They also demonstrated the crystalliza-
tion and structure stability of ␲-TiO
2
[7]. The comprehensive
reviews associated with the fabrication factors, characteriza-
tions, formation mechanism, and the corresponding applications
of TiO
2
-based nanotubes arrays have been also conducted by
Grimes’ group [24]. These methods, other than the hydrothermal
process, are either not suitable for large scale production or not
able to yield very low dimensional, well separated, crystallized
nanotubes [25]. The demonstrated architecture of TiO
2
-based
nanotubes constructed via the hydrothermal treatment is capable

of good crystalline formation and establishment of a pure-phase
structure in one step in a tightly closed vessel.
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2007.07.017
180 H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191
Fig. 1. The scenario of fabrication methods in TiO
2
-based nanotubes.
Among the aforementioned fabrication approaches, both
electrochemical anodic oxidation and hydrothermal treatment
received wide investigations, owing to their cost-effective,
easy route to obtain nanotubes, and the feasibility/availability
of widespread applications. With intention to more detailed
discussions, this paper highlights TiO
2
-based nanotubes synthe-
sized via hydrothermal treatment, for which the corresponding
physical and chemical attributes are tailored to the extensive
applications. It is, therefore, essential to understand the var-
ious factors influencing the characterizations of TiO
2
-based
nanotubes synthesized via hydrothermal treatment. Also, it
should be noted that either the modification of hydrother-
mal treatment or the post-treatment of TiO
2
-based nanotubes
would dominate the corresponding features of TNTs, in other
words, the feasibility of the application is subject to the pre-
treated conditions. Based on extensive literature reviews with

regard to TiO
2
-nased nanotubes, the authors have categorized
five broad groups, characterizations and formation mecha-
nism, the effects of fabrication factors and washing process,
post-treatments, modifications, and applications, which are fur-
ther subdivided into their pertinent studies. Fig. 2 shows the
research scenario of hydrothermal treatment related to the
technical aspects which are further elucidated in the follow-
ing materials. Readers are referred to the listed references
for more detail related to the experimental methodology and
conditions.
2. Characterizations and formation mechanism of TNTs
TiO
2
-based nanotubes, with specific surface area of
400 m
2
g
−1
and 8 nm in diameter, via hydrothermal treatment
was first reported by Kasuga et al. [4] who assigned the obtained
nanotubes for the anatase phase. Their following research also
demonstrated the formation mechanism of nanotubes [11]. The
Fig. 2. Research scenario of TNTs synthesized via the hydrothermal treatment.
Table 2
Proposed chemical structures of TNTs andtheir corresponding lattice parameters
Chemical structure Lattice parameters
Anatase TiO
2

Tetragonal; a = 3.79 nm, b = 3.79,
c = 2.38
N
2
Ti
3
O
7
,Na
2
Ti
3
O
7
,Na
x
H
2−x
Ti
3
O
7
Monoclinic; a = 1.926 nm, b = 0.378,
c = 0.300, β = 101.45
◦
H
2
Ti
2
O

4
(OH)
2
,Na
2
Ti
2
O
4
(OH)
2
Orthorhombic; a = 1.808 nm,
b = 0.379, c = 0.299
H
x
Ti
2−x/4

x/4
O
4
(H
2
O) Orthorhombic; a = 0.378 nm,
b = 1.874, c = 0.298
H
2
Ti
4
O

9
(H
2
O) Monoclinic; a = 1.877 nm, b = 0.375,
c = 1.162, β = 104.6
◦
present debate over the crystal structure of TiO
2
-based nanotube
is among the following: anatase TiO
2
[11,26–28]; lepidocrocite
H
x
Ti
2−x/4

x/4
O
4
(x ∼ 0.7, : vacancy)[29,30];H
2
Ti
3
O
7
/
Na
2
Ti

3
O
7
/Na
x
H
2−x
Ti
3
O
7
[12–15,19,32,33];H
2
Ti
2
O
4
(OH)
2
/
Na
2
Ti
2
O
4
(OH)
2
/Na
x

H
2−x
Ti
2
O
5
(H
2
O) [16,17,20–23,34,35];
H
2
Ti
4
O
9
(H
2
O) [36]. The lattice parameters for each
chemical structure are shown in Table 2. From literature
surveys, the chemical composition of Na
x
H
2−x
Ti
3
O
7
and
Na
x

H
2−x
Ti
2
O
4
(OH) groups were more acceptable than
other structures. As such, the following will emphasize the
characterizations and formation mechanisms of these two
Table 1
Comparisons of current methods in TNT fabrication
Fabrication method Advantages Disadvantages TNT features
Template–assisted method
(1) The scale of nanotube can be moderately
controlled by applied template
(1) Complicated fabrication process
Ordered arrays (powder form)
(2) Tube morphology may be destroyed
during fabrication process
Electrochemical anodic
oxidation method
(1) More desirable for practical applications (1) Mass production is limited
Oriented arrays (thin film)
(2) Ordered alignment with high aspect ratio (2) Rapid formation kinetics is subjected
to the utilization of HF
(3) Feasible for extensive applications (3) Highly expense of fabrication
apparatus
Hydrothermal treatment
(1) Easy route to obtain nanotube morphology (1) Long reaction duration is needed
Random alignment (powder

form)
(2) A number of modifications can be used to
enhance the attributes of titanium nanotubes
(2) Highly concentrated NaOH must be
added
(3) Feasible for extensive applications (3) Difficult in achieving uniform size
H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191 181
structures in terms of some special and novel techniques,
where TiO
2
-based nanotubes are abbreviated as TNTs and not
subject to any structure mentioned above. Even though some
attempts have been dedicated to the formation mechanism
of TNTs, an explicit explanation is unavailable because the
chemical structure of TNTs is still controversial. Moreover,
TNTs were proposed to form either before or after acid
washing treatment; Kasuga et al. [11] tentatively suggested
that the formation of TNTs was achieved after acid wash-
ing, while Peng’s group [12] reported that TNTs can form
during the reaction of TiO
2
with NaOH in hydrothermal
treatment.
2.1. The group of Na
x
H
2−x
Ti
3
O

7
Peng’s group [13] indicated two possible formation mecha-
nisms of H
2
Ti
3
O
7
. In their report, trititanate (Ti
3
O
7
)
2−
sheets
may grow within the intermediate phase, caused by the reaction
between NaOH and TiO
2
. The nanosheets grow with an increas-
ing tendency of curling, leading to the formation of nanotubes.
Also, Na
2
Ti
3
O
7
-like nanocrystal was postulated to form in this
disorder-phase, and single trititanate layer subsequently peeled
off from the nanocrystal and curved naturally likewood shavings
into nanotube. This phenomenon was inferred from the excessive

intercalation of Na
+
between the spaces of crystals. Their other
studies reinforced the afore stated mechanism [14,15] where
the hydrogen-deficiency on the surface of (Ti
3
O
7
)
2−
plates can
provide the driving force (surface tension) for the peeling-off
of (Ti
3
O
7
)
2−
plates and therefore resulting in the layers bent to
form tube morphology. In their reports, the optimum dimension
of TNTs has also been surveyed in terms of energy views. The
number of layers within TNTs was subject to Coulomb energy,
which was induced by the negatively charged (Ti
3
O
7
)
2−
layers.
Coupling energy, resulting from the contributions of unequal dis-

tribution between two sides of (Ti
3
O
7
)
2−
layers and the usual
elastic strain energy of bent crystalline plate, optimize the radius
of TNTs at 4.3 nm. At the same time, an atomic model for TNTs
based on investigations with X-ray diffraction (XRD), high-
resolution transmission electron microscope (HR-TEM), and
selected area electron diffraction (SAED) was also established
[14]. This report demonstrated that the tubes may be constructed
by wrapping a (1 0 0) plane along AA

, as indicated in Fig. 3(b).
Fig. 3(c) illustrates the construction of a nanotube by the dis-
placement of A

with a space of 0.78 nm, and the structure and
cross-sectional view of TNTs are shown in Fig. 3(a) and (d),
respectively.
Special analytic methods, including ion conductivity and
solid-state nuclear magnetic resonance (NMR), have been
employed to investigate the thermal behavior of H
2
Ti
3
O
7

and
the distinguishable phenomenon between structural protons and
trapped water [19]. Based on spectroscopic plots of conductiv-
ity measurements for H
2
Ti
3
O
7
at temperatures of interest (30,
130 and 300
◦
C), a less distributed response at high tempera-
ture was observed. This phenomenon was ascribed to the higher
degree of crystallization in the sample after thermal treatment.
The peaks obtained from NMR analysis for H
2
Ti
3
O
7
after ther-
mal treatment can be exclusively attributed to the contributions
of structural proton and trapped H
2
O. In separate studies, the
amorphous regions can also be observed within TNTs struc-
ture because of defects during the formation process, including
the inappropriate attachment between nanosheets, and the un-
saturation of dangling bonds on the surfaces of lamellar sheets

[37,38].
2.2. The group of Na
x
H
2−x
Ti
2
O
4
(OH)
2
A postulate as to why the TNT structure can be assigned
for the Na
2
Ti
2
O
4
(OH)
2
phase is provided by Yang et al. [16]
where they thought it is impossible for the weak acid H
2
Ti
3
O
7
to exist in concentrated NaOH. Further results with regard to the
dependence of Na/Ti on pH values indicated that TNTs within an
H

+
/Na
+
ratio of 4 can present good stability during hydrothermal
treatment. For the lattice parameter of H
2
Ti
2
O
4
(OH)
2
, the large
elongation along the a axis was ascribed to the layered structure
of the material. Based on electron spin resonance (ESR) mea-
surements, the optical characterizations of dehydrated nanotube
H
2
Ti
2
O
4
(OH)
2
have also been studied by Zhang et al. [18]. They
indicated thedependence of the concentration ofsingle-electron-
trapped oxygen vacancies (g = 2.003) on vacuum dehydration
time increases the visible-light absorption power. This gives
Fig. 3. Structure models of (a) 2 × 2 unit cells of H
2

Ti
3
O
7
on the [0 1 0] projection and (b) a layer of H
2
Ti
3
O
7
on the (1 0 0) plane from which the nanotube is
constructed. AA

and AA

indicate the chiral vectors. Schematic diagrams show (c) the introduction of a displacement vector AA

when wrapping up a sheet to form
a scroll-type nanotube and (d) the structure of tritianate nanotubes. The crystal orientations indicated are the orientations according to the H
2
Ti
3
O
7
layer [14].
182 H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191
Fig. 4. Schematic diagrams: (a) formation process of Na
2
Ti
2

O
4
(OH)
2
and (b)
mechanism for breaking of Na
2
Ti
2
O
4
(OH)
2
[18].
strong support for dehydrated nanotube H
2
Ti
2
O
4
(OH)
2
to be
applied on some technological fields under visible light irradia-
tion.
The formation mechanism of Na
2−x
H
x
Ti

2
O
4
(OH)
2
phase
was also provided by Yang et al. [16], where the swell of TiO
2
particles was considered as the initial stage. Swelling stripes
and the peel-off of granules can subsequently be found based
on the TEM observation, after which tube structure is formed.
The detailed mechanism is as follows: the shorter Ti–O bonds
within TiO
6
units are expected to divide under the concentrated
NaOH solution, and results in irregular swelling. The result-
ing linear fragments would link to each other by O
−
–Na
+
–O
−
bonds to form flexible planar fragments. Nanotubes could be
obtained through the covalent bonding of end groups, as indi-
cated in Fig. 4(a). Fig. 4(b) describes the intralayer composition
of Na
2−x
H
x
Ti

2
O
4
(OH)
2
after the replacement of Na
+
by H
+
during acid washing. This mechanism was further emphasized
by Tsai and Teng [21], who indicated that the replacement
of Na
+
by H
+
cause a peeling-off of individual layers from
TiO
2
particles owing to the variation of the surface charge.
Further supports, including the lattice parameters and some
conclusions obtained from XRD results, related to the assign-
ment for Na
2−x
H
x
Ti
2
O
5
(H

2
O) phase were also reported in this
research. The energy defect caused by dangling bonds on the
TiO
2
layers must be compensated to stabilize the structure. Sub-
sequently, the lamellar TiO
2
bent to form non-concentric tube
structures.
2.3. Other supporting evidence in TNT formation
In Kasuga’s research [11], it was considered that the reaction
of Ti–O–Na with acid would lead to the formation of sheets,
along with a decrease in the length of Ti–O–Ti bonds. The
residual electrostatic repulsion of Ti–O–Na bonds may cause a
connection between Ti–O–Ti sheets and subsequently lead to the
formation of tube structure. The oriented crystal growth pertain-
ing to the formation of TNTs was also indicated by Kukovecz et
al. [39]. Some materials were peeling off from anatase particles,
leaving behind terraces on the surface, and re-crystallizing as tri-
titanate sheets. These sheets subsequently curved into nanoloop,
which was believed to be the seed in the formation process of
TNTs, and the curvature of the loops determined the morphology
of TNT cross sections giving rise to spiral, onion, and multiple-
spiral types. In a separate study, the rolling mechanism from
nanosheets into nanotubes was also reported by Ma et al. [40],
who indicated that the de-intercalation of Na ions caused by
H
3
O

+
substitution would reduce the interaction between lay-
ered sheets. The topmost layer would peel off due to a reduction
in electrostatic interaction with the underlying substrates and
gradually curl up into tube structure. Another study highlighting
the soft chemical reactions also proposed the related formation
mechanism [41]. In this report, Na
2
Ti
3
O
7
, used as the Ti precur-
sor instead of TiO
2,
was capable of synthesizing TNTs without
the presence of NaOH. It was also indicated that [TiO
6
] lay-
ers can hold each other owing to the strong static interaction
between [TiO
6
] units within Na
2
Ti
6
O
13
. The replacement of
Na

+
by H
2
O during hydrothermal treatment would weaken the
static interaction, resulting in the exfoliation of [TiO
6
] layers
from Na
2
Ti
6
O
13
particles. An intrinsic extension existed owing
to the inversion symmetry of these sheets which led to the curling
process into tube structure.
3. Effects of fabrication factors in TNT fabrication
Applied temperature, treatment time, the type of alkali solu-
tion, and the Ti precursor are considered as the predominant
factors in TNT fabrication during hydrothermal treatment. It
has been established that the dependence of morphology and
features of TNTs on hydrothermal conditions significantly dom-
inates the corresponding characterizations of TNTs. Therefore,
it is essential to assemble related results and construct a well-
defined conclusion.
3.1. Applied temperature and treatment duration
Seo et al. [42] revealed that the amount and length of TNTs
gradually increase with applied temperatures (100–200
◦
C),

where the largest specific surface area along with the larger
inner diameter of TNTs emerged at synthesis temperature of
150
◦
C. In a separate study, pore structure of TNTs relevant to
the applied temperature and the concentration of acid-washing,
was also reported by Tsai and Teng [20]. In the case of temper-
atures ranging from 110 to 150
◦
C, the maximum pore volume
and surface area occurred for TNTs synthesized at 130
◦
C.
A reasonable concept was proposed that temperatures lower
H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191 183
than 130
◦
C led to less cleavage of Ti–O–Ti bonds, which
was the initial stage in synthesizing TNTs. Treatment at high
temperature (>130
◦
C) would destroy the lamellar TiO
2
,an
intermediate in the TNTs formation process. Poudel et al. [25]
first related the filling fraction and pressure of autoclave to
the characterizations of TNTs. Either case of filling fraction
or acid washing governs the performance of crystallization,
where the optimum filling fraction (86% to the vessel volume)
and 0.1N HCl were reported to be capable of good crystalline

formation.
3.2. Applied alkali solute and Ti precursors
The effects of NaOH concentration, applied temperature,
and precursors (Degussa P25, anatase and amorphous TiO
2
)
on the TNT formation have also been investigated by Yuan
and Su [43], who concluded that the hydrothermal tempera-
ture at 100–160
◦
C results in the production of TNTs; Nanofiber
was found being H
2
Ti
3
O
7
phase when amorphous TiO
2
was
used as the precursor. Moreover, nanoribbons occurred at the
NaOH concentration of 5–15N under the temperature range
of 180–250
◦
C, which was assigned for the H
2
Ti
5
O
11

(H
2
O)
phase. Nanowires formed exclusively at the solution of KOH and
were indexed as K
2
Ti
8
O
7
, whereas nanowires were obtained in
the LiOH treated samples. Non-hollow nanofibers/nanoribbons
were also reported in an apparently similar hydrothermal proce-
dure [44]. A ribbon-like structure with the width of 30–200 nm
was obtained under the hydrothermal conditions of 10N NaOH
for 24 h at 200
◦
C. These nanoribbons were evidenced to be
anatase TiO
2
[44]. The role of Na atoms in fabrication pro-
cesses has been investigated by Chen et al. [13]. In their results,
TNTs formed exclusively in the presence of Na atom while
nanorods/plates and nanoparticles were observed in the KOH
and LiOH reacted samples.
Many studies indicated that the anatase phase was the pre-
ferred phase with higher surface energy in synthesizing TNTs
[20,45]. This was also confirmed based on the crystalline char-
acteristics [42,46]. Comparatively, Tsai and Teng [21] have
elucidated that rutile phase would be more vigorous than

anatase phase in the rearrangement, which was the intermedi-
ate stage to form TNTs. For the rutile phase as the precursor
of TNTs, the increasing hydrothermal temperature and duration
can result in single-crystalline nanorods with excellent thermal
stability [47].
3.3. The effect of acid washing
Despite Kasuga et al. [11] tentatively proposed that acid-
washing was one step of the formation process of TNTs,
following researches have suggested acid-washing just for the
ion exchangeable process [12,47]. Even though the formation
mechanism is still ambiguous, the acid-washing process appre-
ciably affects the attributes of TNTs owing to the relative amount
of Na and H atoms within TNT structure. Acid washed TNTs
are believed to possess more intercalated H
2
O than non-acid
washed TNTs, and subsequentlyresult in greater weight loss dur-
ing thermal gravimetric analysis (TGA) spectrum [31]. In terms
of the pore structure of TNTs, an optimum concentration of HCl
(0.2N) during the washing process was suggested because the
rapid removal of electrostatic charges caused by high acid con-
centration is detrimental to the formation of TNTs [20]. Their
following research demonstrated the same results where the t-
plot method and density function theory were utilized to explain
the pore structure of TNTs treated by HCl under various pH [23].
Either critical pore diameter or external surface area obtained
from the aforesaid analytic methods responded to the surface
area and pore volume, and evidenced the effect of acid-washing
on the structure of TNTs more clearly. Yang et al. [16] discov-
ered the phenomenon of replacement of Na

+
in Na
2
Ti
2
O
4
(OH)
2
by H
+
. This notion was reinforced by Nian and Teng [22], who
demonstrated a similar behavior in XRD patterns and that the
ratio of peak 110–310 is convinced as being the evidence of the
displacement of Na
+
by H
+
. Similar XRD patterns have also
been demonstrated in other studies, even though they preferen-
tially assigned the obtained TNTs to Na
x
H
2−x
Ti
3
O
7
[48,49].
Weng et al. [48] indicated that hydrogen–TNTs exhibited a

broad peak from 2θ =23
◦
to 25
◦
while another characteristic
peak appear at 28
◦
for sodium–TNTs. Systematic study asso-
ciated with the stability and structure of TNTs as a function
of Na content has also been investigated in detail by Morgado
et al. [49]. This report demonstrated that the interlayer spacing
of TNTs increases with more intercalated Na amount, which
also aids the stability of TNTs during thermal treatment. The
behavior of water re-absorption of TNTs with an abundant Na
amount was also proved based on the TGA experiment. The
crystal composition of TNTs after thermal treatment was deter-
mined by Rietveld analysis, which indicated that TNTs with
low Na content causes crystallization of TiO
2
with anatase
phase and brookite phase. An increase in Na content within
the TNT structure results in another re-crystallization path-
way to form Na
2
Ti
3
O
7
and Na
2

Ti
6
O
13
. The performance of
BET surface area (S
BET
) is also subject to the intercalating
amount of Na atoms, for which the collapse of tube structure
occurred earlier and more drastically for TNTs with a low Na
amount.
4. Post-treatments of TNTs
In many investigations directed towards post-treatments of
TNTs to achieve the activity of TNTs with the intention of
comprehensive applications, post-thermal treatment received
more attention than other treatments. In attempts at the inves-
tigation of the crystalline phase for thermally treated TNTs,
the presence of Na atoms within TNT structure was signifi-
cantly responsible for the corresponding thermal behavior [31].
Yoshida et al. [50] also reported a similar phenomenon where
some nanotubes began to break and condensed into particles of
anatase phase at temperatures higher than 350
◦
C, and others
with a large quantity of Na remained as nanotube. Na atoms
within TNT structure dominate the formation of Na-included
crystallization while proton-TNTs proceed with another re-
crystalline pathway to form anatase phase or even rutile
phase.
184 H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191

4.1. Phase structure and pore structure of TNTs after
thermal treatment
Investigations pertaining to the overall effect of thermal
treatment on TNTs have been conducted by many researches.
Predominant phases including TiO
2
(brookite), TiO
2
(anatase),
TiO
2
(rutile), Na
2
Ti
3
O
7
,Na
2
Ti
6
O
13
, etc. for TNTs after thermal
treatment have been demonstrated. Suzuli and Yoshikawa [51]
found the existence of TiO
2
(B) free of anatase after the thermal
treatment of TNTs while Armstrong et al. [52] also observed
TiO

2
(B) for their nanowires after thermal treatment of TNTs
at 400–600
◦
C. Poudel et al. [25] indicated that the rutile phase
begin to crystallize at 800
◦
C, well below the transformation tem-
perature of 925
◦
C for bulk anatase TiO
2
nanopowder. Also, a
change from nanotubes to nanowire morphologywas observed at
the annealed temperature of 650
◦
C. Further comparisons in this
research also present that TNTs are less stable under oxygen than
under vacuum, although still more stable than TNTs fabricated
by electrochemistry anodic oxidation. In other reports, the onset
of anatase to rutile transformation was also reported at 700
◦
C
by Yu et al. [53], while another research provided it at 900
◦
C
[20]. Tsai and Teng [20] also indicated that the temperature for
anatase torutile transformation was relevant to the synthesis tem-
perature of TNTs where such transformation occurred at 900
◦

C
while TNTs was synthesized at 130
◦
C. When TNTs were cal-
cined at 600
◦
C, Na-containing species of Na
2
Ti
9
O
19
emerges
and thereafter transforms as Na
2
Ti
6
O
13
and TiO
2
at 800
◦
C [31].
Tsai and Teng [21] suggested that Na
2
Ti
6
O
13

within a tunnel
structure can behave as a high thermal insulation with chemical
stability; therefore, it can be used as potential adiabatic mate-
rials. The result was further evidenced in the following reports
[49,50,54]. While thermal temperature is higher than 300
◦
C,
amorphous phase can be observed and is ascribed to the dehy-
dration of the intralayered OH group within TNTs [18]. Further
explanation in terms of mass-transport of atoms within TNTs
during thermal treatment was also demonstrated. In this report,
it was indicated that the morphology was changed to a rod-like
one for which the length was relevant to the amount and dis-
tribution of defects, contributed by the dehydration of the OH
group. Another contribution provided by Nian and Teng [22]
indicated that the rod formation was ascribed to the oriented
attachment of adjacent TNTs together with the local shrinkage
of the TNTs during thermal treatment. Systematic studies con-
cerning the reversible transitions of crystal phase by different
treatments have also been conducted [55]. In fact, the crystal
phase and morphology change of TNTs after thermal treatment
are significantly relevant to the amount of Na atoms intercalated
with TNTs, as indicated in Fig. 5.
The textural parameters from the adsorption–desorption
isotherm data for TNTs after thermal treatment were also exam-
ined by Yu et al. [56]. The specific surface and pore volume
decrease with increasing calcination temperature, suggesting the
collapse of tube structure. They also indicated that the advantage
of high pore volume and specific surface area can be preserved
until the calcinations temperature reached 600

◦
C. However, the
pore size of TNTs increases to 44.8 nm at 700
◦
C and then dra-
matically decreases to 8.2 nm at 800
◦
C; This phenomenon was
Fig. 5. Possible crystal phases and morphologies of TNTs after thermal treat-
ment.
attributed to the collapse of small pores inside TNTs and the
growing crystallization of TiO
2
. In another conclusion [20],
the high porosity in TNTs was also reported to disappear after
thermal treatment at 600
◦
C. Beside the aforementioned investi-
gation, the optical property of thermally treated TNTs was also
studied by Wang et al. [33]. The hydration and nano-sized effect
caused the blue shift ofTNTs whose absorption edge was 342 nm
while that of bulk anatase TiO
2
was 385 nm. The visible absorp-
tion of thermally treated TNTs resulting from the growth of
new crystallization, Ti
5
O
9
and anatase TiO

2
was enhanced with
increasing temperatures of 400–600
◦
C.
4.2. Other post-treatments of TNTs
While thermal treatment of TNTs displays beneficial effects
on photocatalytic ability, it is detrimental to the physical aspects
of TNTs such as BET surface area and pore volume. Therefore,
researchers are also looking into alternative methods to increase
the activity of TNTs without the undesirable effect of pore block-
age to avoid the elimination of surface OH group and to stabilize
tube morphology during thermal treatment. However, so far, far
too few post-treatments were successful or well developed.
Bavykin et al. [32] have investigated the structural change of
acid-immersed TNTs after a series of treatment periods. They
indicated there were three stages for structural change of TNTs
during the immersion process; (1) erosion and disruption of TNT
structure, (2) the formation of rutile nanoparticles and H
2
Ti
3
O
7
phase, and (3) stable rutile phase along with trace amount of
TNTs were present. Meanwhile, the results derivated from that
of concentrated acid and thermal treatment were ascribed to the
lower rate of phase change, and this report suggested that these
can be promising candidates to obtain rutile phase. The post-
hydrothermal treatment of TNTs has also been investigated by

Nian and Teng [22]. The characterization of treated TNTs is sub-
ject to the applied pH conditions; only anatase phase appears at
pH 2.2 while anatase along with brookite can be observed at pH
8.2. Rod morphology was found exclusively for TNTs treated
at pH 5.6, which was also assigned for anatase phase. Crystal
enlargement with pH values is anisotropic and the condition at
pH 5.6 makes the maximum enlargement degree result in rod
formation. Similar research emphasizing the phase structure,
morphology, and pore structure has also been investigated [56].
In this research, fiber-like structure with anatase phase can be
observed after post-hydrothermal treatment. Furthermore, the
growth of TiO
2
crystallites with increasing post-hydrothermal
treatment time was evidenced to be responsible for a small distri-
bution of pore size, a decrease in pore volume and average pore
H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191 185
Table 3
Recent studies concerning the morphology and crystal phase of TNTs after post-treatment
Post-treatment Reference Observed results
Post-thermal treatment
Yoshida et al. [50] Some nanotubes began to break into particles of anatase phase at temperature higher than
350
◦
C while the others remained as nanotube with the presence of a large quantity of Na
Suzuli et al. [51] The existence of TiO
2
(B) free of anatase during thermal treatment of TNTs
Armstrong et al. [52] TiO
2

(B) with nanowires morphology after thermal treatment of TNTs at 400–600
◦
C
Poudel et al. [25] Rutile phase begin to crystallize at 800
◦
C; nanotubes to nanowire morphology was
observed at the annealed temperature of 650
◦
C
Tsai and Teng. [20] Anatase to rutile transformation was reported at 900
◦
C
Yu et al. [53] Anatase to rutile transformation was reported at 700
◦
C
Sun and Li [31] Na
2
Ti
9
O
19
emerges at 600
◦
C and thereafter transforms as Na
2
Ti
6
O
13
and TiO

2
at 800
◦
C
Zhang et al. [18] Amorphous phase emerges at thermal temperature higher than 300
◦
C
Yu et al. [56] Pore volume and specific area of TNTs can be preserved until the calcinations temperature
achieved 600
◦
C
Wang et al. [33] The visible absorptions of thermal treated TNTs were enhanced with increasing
temperatures of 400–600
◦
C
Post-hydrothermal treatment Nian and Teng [22] The pH values during hydrothermal treatment dominates the corresponding behavior
Yu et al. [56] Fiber-like structure with anatase phase was observed; Increasing treatment time dominate
the pore structure of TNTs
Acid immersion process Bavykin et al. [32] Stable rutile phase formed owing to the low rate in phase change
Electrodeposition process Kim et al. [58] TNTs were fabricated as thin film without the presence of Na atoms
Hot filament chemical vapor deposition Godbole et al. [60] Different treatment conditions results in the different crystal phase
diameter. Regular multi-layer films of TNTs have been fabri-
cated in a sequential layer-by-layer assembly with polycations
[57]. An approximately equal amount of TNTs was deposited
for each layer pair in the fabrication process, which provided
a criterion, as far as this deposition method was concerned,
for the stepwise and regular film growth process. For another
deposition method, TNTs coated on silicon substrates by the
electrodeposition process has also been demonstrated by Kim
et al. [58,59]. Their observation indicated that electrodeposited

coating resulted in negligible or zero concentration of sodium;
further study based on X-ray photoelectron spectroscope (XPS)
determinations showed that the reduction of strongly bonded
sodium can be achieved by electrodeposition process while acid
treatment just provided the ability to remove weakly bonded
sodium. A point worthy of mentioning is that TNTs can inherit
its tube morphology via electrodeposition process as a thin
film, which is desirable for practical applications. The results of
their following research associated with the characterizations of
coated TNTs after some processing was also demonstrated [60].
Coated TNTs processed by hot filament chemical vapor deposi-
tion (HF-CVD) under various conditions presents significantly
different results. Atmospheric/vacuum processing result in the
rutile and anatase phase; no characteristic phase was observed
after plasma treatment. In the case of H
2
/CH
4
mixing gas, some
composite phases can be observed including rutile phase (TiO
2
),
non-stoichiometric phases (Ti
2
O
3
and Ti
3
O
5

), titanium carbide,
and extensive carbon nanowires and nanotubes. All the afore-
mentioned studies concerning the post-treatments of TNTs are
shown in Table 3.
5. Modifications of hydrothermal treatment
In spite of the previous discussions in favor of the synthesis of
TNTs for its excellent morphology, some limitations for TNTs
as advanced materials emerge owing to their low crystalline
content. To inherit or regain the activity from the precursor,
further modifications in hydrothermal treatment were required.
Also, with an aim to shorten the long duration in synthesizing
TNTs, some assisted methods have been developed to enhance
the formation kinetic of TNTs. The authors have categorized
two broad groups, namely, chemical modification and physical
modification to discuss related reports, as indicated in Table 4.
5.1. Chemical modification
Nanorods can be formed by surface modification of n-
octadecytrichlorosilane (OTS) in hydrothermal treatment [61].
A possible explanation was also provided that OTS can
hydrolyze then be adsorbed onto the surface of TNTs, along
with the coverage of hydrophobic group onto the surface of
TNTs. The resulting TNTs can aggregate themselves to form
thinner rods, and further aggregation can result in thinner ones.
Another study indicating the presence of Zn
2+
in hydrothermal
treatment would cause the formation of layered H
2
Ti
2

O
5
(H
2
O)
nanosheets [34]. TNTs with ultrahigh crystallization can be
obtained after H
2
O
2
treatment under reflux at 40
◦
C for 4 h
[45]. This report indicated that the oxygen vacancies can be
compensated by H
2
O
2
, as being supported by some measure-
ments including XRD, HRTEM, and photoluminescence (PL).
Especially the blue shift of H
2
O
2
-modified TNTs suggested the
recovery of oxygen vacancies of TNTs after treatment with
H
2
O
2

. Meanwhile, the intensity of anatase phase for H
2
O
2
-
modified TNTs can be drastically enhanced due to the presence
of H
2
O
2
. Another related study demonstrated that the presence
of H
2
O
2
in NaOH solution at a temperature of 220
◦
C for 48 h
can be developed as the ordered array of titanate with aspect
ratios of 20,000 [62], which was the first report regarding the
development of titanate nanowire arrays via hydrothermal treat-
186 H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191
Table 4
Recent techniques used to modify hydrothermal treatment
Reference Techniques used Contributions
Chemical modification
Zhang et al. [61] The presence of n-octadecytrichlorosilane during
hydrothermal treatment
The formation of nanorods
Song et al. [34] The presence of Zn

2+
during hydrothermal treatment The formation of nanosheets
Khan et al. [45] The presence of H
2
O
2
under refluxing at 40
◦
C for 4 h The intensity of anatase was drastically enhanced
Kim et al. [58] Electrophoretic deposition The sodium content within TNTs was reduced
drastically in electrodeposition process
Zhao et al. [62] The presence of H
2
O
2
during hydrothermal treatment at
220 for 48 h
The formation of ordered arrays of TNTs
Weng et al. [48] Na
2
Ti
3
O
7
was used as the Ti precursor during
hydrothermal treatment
TNTs can be obtained without the presence of NaOH
Kukovecz’s group [63,64] The presence of Na
2
S during hydrothermal treatment Resulting in the formation of CdS nanoparticles/TNTs

nanocomposites
Ren et al. [65] The presence of thiourea and urea during hydrothermal
treatment
The formation of S–TiO
2
and N–TiO
2
with dandelion
morphology
Physical modification
Zhu et al. [66] Sonic-assisted hydrothermal treatment The formation kinetics of TNTs was enhanced
Ma et al. [67] Sonic-assisted hydrothermal treatment The formation kinetics of TNTs was enhanced
Wang et al. [68] Microwave-assisted hydrothermal treatment The formation kinetics of TNTs was enhanced
Wu et al. [27,28] Microwave-assisted hydrothermal treatment The formation kinetics of TNTs was enhanced
ment. These authors also assumed the nanowires grow along a
perpendicular direction to form arrays. Soft chemical reaction
has also been reported where TNTs can be found without the
presence of NaOH when Na
2
Ti
3
O
7
instead of TiO
2
was used
as the Ti precursor [48]. In their demonstration, TEM observa-
tions and pore size distribution presented that TNTs exhibited
excellent homogeneous distribution. Also, the length of TNTs
increases with a prolonged treatment period.

Kukovecz’s group [63] has modified the precursor as a mixing
solution of Na
2
S/NaOH to synthesize CdS/TNTs nanocompos-
ites. Two steps were first reported in this fabrication, but they
made a modification for the fabrication to be conducted as a
one-step process [64]. They indicated that the uniform particle
size and high tube coverage of CdS nanoparticles were con-
tributed by the homogeneous solution phase of the Cd–EDTA
complex. The measured CdS diameter in these two studies fell
into the range of 3–9 and 2.4–8.4 nm, respectively. A separate
study showed that doped S–TiO
2
and N–TiO
2
with dandelion
morphology can also be fabricated in the presence of thiourea
and urea during hydrothermal treatment [65]. These samples
exhibited excellent stability and even subjected their slurry to
ultrasonication for 1 h, in which the strong chemical bonding
between contacting lateral surfaces at the inner ends of rods was
inferred to contribute to stability. The doped TiO
2
nanodande-
lion with rutile phase also demonstrated photocatalytic activity
to methylene blue degradation.
5.2. Physical modification
It is inevitable to allow at least 20 h for hydrothermal treat-
ment with intention to achieve a high level of crystallization in
TNTs, so it is important to consider other effective candidates to

shorten the synthesis duration. However, so far, few researches
have been dedicated to rapid kinetics in TNT formation. Zhu
et al. [66] have proposed a technology coupled with sonication
and hydrothermal treatment in which the synthesis duration is
shortened from 20 to 4 h. A similar result has been evidenced by
Ma et al. [67]. To best of acknowledge, Zhang’s group [68] dis-
covered that TNT structure can be rapidly achieved with the aid
of microwave irradiation, and a similar result was subsequently
revealed by Wu et al. [27]. The effects of treatment time, concen-
tration of NaOH, applied irradiation power, and Ti precursors
on the characterization of TNTs were subsequently investigated
[28]. Both reports indicated that the chemical structure of TNTs
is assigned for anatase TiO
2
. Regarding the effect of irradiation
power on the formation of TNT structure, the formation kinet-
ics is only enhanced under optimum irradiation power while
overload of that would resolve and destroy the crystallization
[28]. Potassium titanate nanowires have also been fabricated
by microwave-assisted hydrothermal treatment conducted by
Zhang’s group [69]. A plausible explanation has also been pro-
posed that microwave is capable of changing the polarization of
hydroxyl species on the surface of the solid, facilitating reaction
between solid and liquid.
6. Applications of TNTs and TNT-derived materials
Of the TNT materials being developed for various applica-
tions, many investigations have emphasized photocatalysis. The
synthesized TNTs, unfortunately, generally do not inherit pho-
tocatalystic ability from the anatase phase of TiO
2

. A suitable
and feasible method to regain the photocatalytic ability is the
post-thermal-treatment, and many studies in this regard have
acquired well-established conclusions. Moreover, applications
on support/carriers, ion-exchange/adsorption, photochemistry,
dry sensitized solar cells, and other prominent applications
are also discussed in the following materials and compiled in
Table 5.
H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191 187
Table 5
Applications of TNTs on versatile aspects
Reference Treatment Applications Performance
Support/carrier
Wang et al. [35] Support of benzoic acid Dispersion capacity Benzoic acid can dispersed as monolayer dispersion on the
surface of TNTs with the utmost capacity of 0.55 g BA g
−1
TNTs
Idakiev et al. [74] Au-supported TNTs WGS reaction Reaction rate is increased than that of Au/Al
2
O
3
by a factor
of 4
Chien et al. [75] Pt/Au-supported TNTs CO
2
hydorgention Reaction rate increased than that of Pt/Au-supported TiO
2
by a factor of 1–30
Tsai and Tang [20] Cu-supported TNTs/thermal
treated

NO conversion Reaction rate is increased than that of P25 TiO
2
by a factor
of 4
Nakahira et al. [77] Pt-entrapped TNTs HCHO conversion Pt/TNTs posses the comparative photocatalytic ability with
TiO
2
Photocatalytic degradation
Yu et al. [53] Thermal treated TNTs Acetone Reaction rate of treated TNTs at 300–600
◦
C is increased
than that of P25 TiO
2
by a factor of 3–4
Xu et al. [71] Zn surface-doped TNTs Methyl organic Reaction rate of thermal treated Zn/TNTs (400–500
◦
C) is
increased than that of TiO
2
nanoparticles by a factor 2–3
Zhang et al. [17] Thermal treated TNTs Propylene Reaction rate of treated TNTs is inferior to that of P25 TiO
2
Song et al. [34] H
2
Ti
2
O
5
(H
2

O) nanosheets Methyl organic Reaction rate is similar to that of TiO
2
but larger than that
of ZnO by a factor of 1.5
Zhu et al. [55] Thermal treated TNTs Surforhodamine Reaction rate of TNTs is larger than that of P25 TiO
2
by a
factor of 2
Khan et al. [45] H
2
O
2
modified TNTs Trimethylamine Reaction rate of H
2
O
2
–TNTs is larger than that of TNTs by
a factor of 2
Yu et al. [53] Thermal treated TNTs Aceton Reaction rate of TNTs treated at 200
◦
C for 7 h is larger
than that of P25 TiO
2
by a factor of 1.5
Gao et al. [72] Thermal treated TNTs Pentachlorophenol Reaction rate TNTs treated at 400 is larger than that of P25
TiO
2
by a factor of 1.5
ˇ
Stengl et al. [70] Thermal treated TNTs 4-Chlorophenol The degradation potential is inferior to that of P25 TiO

2
Nakahira et al. [36] Pure TNTs HCHO Reaction rate of TNTs is larger than that of P25 TiO
2
by a
factor of 1.6
Ion exchangeable and adsorption
Sun and Li [31] None Co
2+
,Cu
2+
,Ni
2+
,NH
4
+
To verify the feasibility of TNTs as a ion-exchangeable
materials
Umek et al. [78] None NO
2
adsorption NO
2
can be reduced as NO in the presence of Na
+
Photochemistry and electrochemistry
Li et al. [84] None Lithium ion battery Initial discharge capacity is larged than that of TiO
2
electrode by a factor of 30–50
Other pioneering application
Lin et al. [46] Sulfated–TNTs Esterification reaction Reaction rate was increased by a factor of 5
Miao et al. [86] Ag/AgCl–TNTs Photochromism Ag/TNTs exhibited multicolor photochromism

Kasuga [82] Ca–TNTs Biocompatibility New bone generate after 7 day implantation in rat
Kim et al. [59] Electrodeposition
process/thermal treated TNTs
Dry-sensitized solar celles Photocurrent density of TNTs film annealed at 500
◦
Cwas
15.67 mA cm
−2
, which was larger than that of TNTs films
fabricated doctor-blade method by a factor of 10
Hu et al. [81] Pd supported on carbonized
TNTs
Conductivity Conductivity is increased than that of Pd/C by a factor of
1.5–3
He et al. [83] Ag-supported/TiO
2
/TNTs Conductivity Ag/TNTs improve the reversibility capacity and the cycling
stability of pure TNTs
Dominko et al. [54] TNTs-derivate: Na
2
Ti
6
O
13
Lithium ion battery To verify the feasibility of Na
2
Ti
6
O
13

as a new negative
electrode
Yu and Zhang [80] Vanadium oxide/titanate Capacitance The electrochemical capacitor of composite is larger than
that of V
2
O
5
Kasuga [82] Acid-treated TNTs Conductivity Conductivity is increased by a factor of 50–100
Tokudome and Miyauchi [79] N-doped TNTs Band gap determination The refractive indices are lower than that of a
polycrystalline anatase TiO
2
thin film
6.1. Photocatalysis
Regarding the photo-degradation of propylene, the effect of
annealing temperature on the photocatalysis ability of TNTs
was revealed by Zhang et al. [17]. TNTs treated at 300
◦
C
possessed the best photocatalytic ability among the thermally
treated TNTs; however, all of them presented inferior perfor-
mances to that of Degussa P25 TiO
2
. The same result was
188 H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191
also demonstrated by
ˇ
Stengl et al. [70] where they derived
titanium nanorod from the post-thermal treatment of TNTs
and investigated the corresponding photocatalytic ability for 4-
chlorophenol degradation. They indicated that even though the

photocatalytic potential of titanium nanorods was inferior to
that of commercial Degussa P25 TiO
2
, the titanium nanorods
still exhibited good ability toward the 4-chlorophenol degrada-
tion owing to its high crystallization. Yu et al. [56] have also
examined the photocatalytic oxidation of acetone over TNTs
under thermal treatment (300–700
◦
C), which presented bet-
ter photoability than commercial P25 TiO
2
owing to the better
pore volume and surface area. When the calcination temperature
exceeds 700
◦
C, the photocatalytic ability disappear because of
the absence of anatase and the decrease in pore volume and sur-
face area. A similar study has also been presented by Xu et al.
[71] where the degradation of methyl organic material was used
as an indicator for the photocatalytic potential of Zn surface-
doped TNTs. They assigned the low photoactivity of Zn/TNTs
calcined at 300
◦
C for the uncompleted complex decomposition
on nanotube surface. The enhanced photoactivity in this case
was ascribed to the Zn ions facilitating the charge separation,
and also the larger surface area and pore size of TNTs. In a
separate study, the calcined TNTs at 400
◦

C has been evidenced
to be more abundant in OH concentration than TiO
2
/SiO
2
[72],
which also support the feasibility of TNTs being applied on the
photocatalysis. Furthermore, they indicated that both extended
capacity of UV-light absorption and large specific surface of
TNTs were predominant factors for the excellent photocatalytic
performance in pentachlorophenol degradation.
Anatase TiO
2
nanofibers can also be obtained from the
hydrothermally post-treatment of TNTs [56]. This report
revealed the photocatalytic potential of anatase TiO
2
nanofiber
for acetone degradation together with CO
2
formation. The pho-
tocatalytic performance was also indicated to exceed that of
the commercial P25 TiO
2
owing to the demonstrated larger
surface area, smaller crystallite size, and higher pore volume.
The photocatalytic ability of TNTs and H
2
O
2

-modified TNTs
with ultrahigh crystalline content for trimethylamine degrada-
tion have also been examined [45], where the oxidant efficiency
of modified TNTs exceeded that of TNTs by a factor of 2. This
phenomenon can be attributed to the compensation of the oxygen
vacancy because of H
2
O
2
modification.
6.2. Support/carriers
Hodos et al. [63] communicated the first successful pho-
toactivation of TNTs by CdS particles. Hsu et al. [73] and
Kukovecz et al. [64] have reported the related synthesis methods
of CdS/TNTs, but did not apply such nanocomposites on some
fields. Idakiev et al. [74] also studied the fabrication of Au-
supported TNTs and the feasibility on water–gas shift reaction
(WGS reaction). The performance of WGS reaction over Au-
supported TNTs was enhanced by as much as four times that
of Au/Al
2
O
3
. Excluding the contribution from Au particles, the
enhanced activity for Au-supported TNTs may be attributed to
the perimeter interaction between Au particles and TNTs, the
weak acidity contributed by TNTs, and the specific structure
of TNTs. However, they also found that part of Au particles
inserted into the tube hollows would shelter the active sites. In
another study, conducted by Chien et al. [75], Pt/Au nanosize

particles supported on TNTs was used to investigate CO
2
hydro-
genation and CO oxidation. TNTs subjected to Cu impregnation
was also applied to examine the catalytic ability on NO conver-
sion [20]. Comparing the catalytic ability of Cu/TNTs to that of
Cu/TiO
2
, this report ascribed the excellent catalytic performance
of Cu/TNTs to the thorough dispersion of Cu on the surface of
TNTs and the high surface area of TNTs. TNTs was also used
as the support of Pd particles to investigate the double-bonded
migration reaction [76]. Pt-entrapped TNTs based on decompo-
sition of HCHO was also investigated by Nakahira et al. [77].
In a separate report where TNTs were used as the carrier of
benzoic acid (BA) [35], BA molecules could be dispersed in
monolayer on the surface of TNTs and carboxylate species could
form owing to the reaction between the carboxylic acid function-
ality and hydroxyl groups of TNTs. Also, the utmost monolayer
dispersion capacity was demonstrated as being 0.55 g BA g
−1
TNTs.
6.3. Ion-exchangeable and adsorption
Sun and Li [31] first investigated the ion-exchangeable ability
of TNTs where the characterizations of metal-substituted TNTs
were influenced by the intercalation of transition metals. The
intercalation of transition-metal-ions into the lattice of TNTs
was ascribed to the electrostatic interactions between the nega-
tively charged host lattice and positively charged cationic ions.
Meanwhile, they also indicated that UV/vis spectrums of Co

2+
,
Cu
2+
, and Ni
2+
-substituted TNTs demonstrated broad and strong
absorption in the visible-metal range owing to the d–d transition
of these transition-metal ions. This feature is believed to possess
a positive impact on some photo-related fields. Regarding the
application of TNTs on adsorption, the impact of structure and
morphology on NO
2
adsorption over nanotubes and nanoribbons
has been reported by Umek et al. [78]. In their electron para-
magnetic resonance (EPR) determinations, physissorbed NO
2
molecules with a trace amount of NO were observed in the case
of nanotubes, while NO dominated in the case of nanoribbons.
They indicated that Na atoms along with the hydrolyzed surface
of nanoribbons can catalyze NO
2
, leading to the formation of
NO
3
and NO. On the other hand, nanotubes with a lower amount
of Na atoms preferentially provide sites for NO
2
adsorption and
few opportunities for NO

2
catalysis.
6.4. Photochemistry and electrochemistry
Modified N-doped TNTs was demonstrated by Tokudome
and Miyauchi [79], in which the band-gap of N-doped TNTs
was reported as 3.17 eV while that of pure anatase and TNTs
were 3.22 and 3.42 eV, respectively. The enhanced attributes,
both low-reflective and transparent, were reported to be due to
the inner cavities of the nanotubes and void spaces between nan-
otubes. Further support was also provided by the degradation of
gaseous isopropanol over N-doped TNTs being feasible under
H H. Ou, S L. Lo / Separation and Purification Technology 58 (2007) 179–191 189
the illumination of 410–500 nm. In a separate study, vanadium
oxide/titanate composite nanorods have also been fabricated
to investigate the corresponding electrochemical capacitance
[80]. This report indicated that the composite materials were
orderly grown together in the form of bundles 10–20 ␮min
length and 100–300 nm in diameter. Illustrated cyclic voltam-
metric curves indicated that the electrochemical capacitance
and voltammetric current of the composites nanorods were bet-
ter than that of pure V
2
O
5
. Conductivity of TNTs was also
enhanced by carbonization treatment [81], where the enhanced
performance of Pd/TiO
2
C in conductivity was ascribed to the
carbonization and efficient mass transport on the surface of

TNTs. Kasuga et al. [82] has also highlighted acid treatment
of TNTs on the electric conductivities, where proton-TNTs,
phosphoric treated TNTs, sulfuric treated TNTs, and perchlo-
ric treated TNTs were 1.6 × 10
−4
, 1.4 × 10
−2
,8× 10
−3
, and
1.6 × 10
−2
Scm
−1
, respectively. This result indicated that the
involvement of surface modification dominated the conductiv-
ity features, which was believed to be related to the dissociation
degree of applied acid. Ag-modified TNTs were also reported
by He et al. [83], in which the surface electronic conductivity
of Ag/TNTs can be improved more completely than bare TNTs.
As such, Ag-modified TNTs significantly decreased cell polar-
ization along with the enhancement of reversible capacity and
cycling stability of the bare TNTs.
Zhang’s group [84,85] first investigated the electrochemical
properties of anatase TiO
2
nanotube, and found its promising
behavior in lithium intercalation batteries. In their study, a high
discharge rate capability and excellent cycling stability of TNTs
were observed. Based on the examination ofcolumbic efficiency,

lithium intercalation and its efficient release from TNTs could
also be found in the layered wall structure of TNTs. This pro-
posal was supported in that the larger interlayer distance within
TNTs provide a promising channel for lithium ion intercalation
and release reversibly [47].Na
2
Ti
6
O
13
, a TNT-derived mate-
rial, within tunnel structure has also been evidenced to provide
accommodations for lithium insertion. Based on galvanostatic
measurements, such insertion can raise the efficiency of lithium
ion batteries if applied on lithium-intercalated Na
2
Ti
6
O
13
as the
negative electrode in the field of lithium [54].
Another promising application in the solar energy related
field was reported by Kim et al. [59]. In this study, TNT film
was fabricated on F–SnO
2
coated glass (FTO) via electrophotic
deposition. The photocurrent densities of the dye-sensitized
solar cells gradually increased with the annealing temperature
of interest (450–500

◦
C). The decrease in photocurrent densi-
ties for temperatures over 500
◦
C was attributed to the thermal
limitation of FTO substrate and the decline of surface area of
TNTs. Another conclusion demonstrated in this paper is that the
sodium containing TNTs and poor interfacial adhesion between
TNTs and FTO substrate can also cause low photocatalytic pho-
tocurrent density.
6.5. Other pioneering applications
The application of sulfated TNTs on the esterification reac-
tion was exhibited by Lin et al. [46]. Based on the observation of
ester yield, the esterification reaction rate of sulfated TNTs was
reported to be larger than that of anatase TiO
2
powder by a factor
of five. Another study querying the application of Ca–TNTs was
also developed where it was used for bone repair in filling defec-
tive areas of bones [82]. Newly formed bone was found around
Ca–TNTs after being implanted in the femur of a rat for 7 days.
This phenomenon indicated that Ca–TNTs induced excellent
bone tissue regeneration at implantation. In a separate study, the
modified Ag/AgCl–TNTs were used as photochromism mate-
rials, which gained prominence in smart window, displays, and
optical memories [86]. In this report it was concluded that multi-
color photochromism corresponding to that of incident light was
present in the case of Ag modified TNTs. This behavior, which
subsequently led to either permanence of presented colored sam-
ples for several days under fluorescent light or bleach by UV

irradiation, was attributed to the improvement of homogeneous
size distribution and photochromic features.
7. Concluding remarks
In this review, an extensive spectrum of hydrothermal
TNTs have been demonstrated where five categorizations have
been classified, including (a) characterization and formation
mechanism, (b) fabrication factors, (c) post-treatment, (d) mod-
ifications and (e) applications. In spite of many studies having
attempted to investigate the chemical structure and formation
mechanism of TNTs, it is still ambiguous and leaves much space
to explore and explain. Post-treatment, believed to improve the
activity of TNTs, may, on the downside, adversely affect its
physical characterizations. It is, therefore, the opinion of the
authors that novel and promising post-treatment or modifying
techniques should be developed further, as these techniques can
enhance the activities of TNTs while at the same time ensuring
that the techniques do not compromise its physical characteriza-
tions. Furthermore, the modification of hydrothermal treatment
also opens new perspectives in the investigation of enhanced for-
mation kinetics and the chemical/physical attributes of TNTs.
From literature surveys, so far, few studies have been dedicated
to this aspect and it should be another potential aspect in TNT
investigations. TNTs with high surface area, ion exchangeable
ability, and photocatalytic potential present an attractive avenue
and is an ideal candidate in extensive applications. In fact, the
versatility and feasibility of TNTs on practical applications have
been demonstrated, and are still in the ascendant.
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